Assertions in unix/modffi should raise a ValueError instead

[luizfeliperj]

I believe that these lines:

micropython/ports/unix/modffi.c

Line 489 in 4625f97

assert(n_kw == 0);

and

micropython/ports/unix/modffi.c

Line 490 in 4625f97

assert(n_args == self->cif.nargs);

should raise a ValueError instead of an assertion.

They are only checked on debug builds of micropython for unix and generates a core dump when tripped. It is really difficult to find out the error when this happens, raising a ValueError instead would print a stack trace with more debug information.